The Effects of NdF₂ on Current Efficiency of Nd Extraction from NdF₃-LiF-Nd₂O₃ Melts

Abstract

In this paper, the cyclic voltammetry was applied to investigate the electrochemical reduction processes of Nd(III) ions in NdF₃-LiF melts with or without excessive metal Nd. Equilibrium experiments were carried out in NdF₃-LiF melts with excessive spheric metal Nd to investigate the relationship between the NdF₃:LiF mass ratio and NdF₂ concentration. Electrolysis experiments were performed in NdF₃-LiF-Nd₂O₃ melts with different NdF₃:LiF mass ratios to explore the relationship between the NdF₂ concentration and the current efficiency.

The results indicated that Nd(III) ions in the melts were reduced in two steps, i.e., Nd(III)→Nd(II) and Nd(II)→Nd(0). NdF₂ could be formed by the comproportionation reaction between Nd(III) and Nd(0) and could stably exist in NdF₃-LiF melts containing metal Nd(0). NdF₂ mass concentration in the melts decreased from 45.5% to 36.4% with the increase of NdF₃-LiF mass ratio from 7:3 to 9:1 in NdF₃-LiF melts containing excessive spheric metal Nd, which resulted in a higher current efficiency during the electrolysis. And the highest current efficiency of about 50% for Nd extraction has been obtained by electrolysis in NdF₃-LiF (9:1 mass ratio) melts with Nd₂O₃ (2%, mass concentration) at 1423 K.

1. Introduction

Currently, NdF₃-LiF-Nd₂O₃ melts are mainly used to extract neodymium (Nd) by molten salts electrolysis due to their higher current efficiency than that of chloride melts, such as NdCl₃-KCl melts. However, the current efficiency for Nd extraction from the NdF₃-LiF-Nd₂O₃ melts is about 70%–80%, lower than that of aluminium electrolysis (about 92%). According to professor Zhu's research¹⁾, the lower current efficiency for Nd extraction from NdCl₃-KCl melts is partly caused by Nd consumption and corresponding to Nd(II) formation by the comproportionation reaction between the Nd(0) and Nd(III). A. Novoselova et al.²⁾, S.Vandarkuzhali et al.³⁾ and Hajimu Yamana et al.⁴⁾ have confirmed that the Nd(III) ions in the NdCl₃-KCl melts are reduced to Nd(0) through two consecutive steps, i.e., Nd(III)→Nd(II) and Nd(II)→Nd(0). Therefore, they have concluded that the Nd(II) ions can stably exist in molten LiCl-KCl-NdCl₃ at 773 K⁴⁾ and LiCl-KCl-CsCl-NdCl₃ melts after electrolysis at 810–840 K²⁾. But so far the existence of stable Nd(II) ions in fluoride melts is ambiguous. The cathodic processes of Nd(III) in fluoride melts revealed by C. Hamel et al.⁵⁾ showed that Nd(III) ions are reduced to Nd(0) in a one-step process in LiF-CaF₂ melts. E. Stefanidaki et al.⁶⁾ have also shown that Nd(III) ions are reduced to Nd(0) via a three-electron reaction in NdF₃-LiF-Nd₂O₃ melts. But in our previous researches^7,8), we have found that the Nd(III) ions in the NdF₃-LiF melts are reduced through two consecutive steps, similar to the reduction process in the chloride melts. So in this paper, we further investigated the existence of Nd(II) ions in NdF₃-LiF melts with excessive spheric metal Nd. The effect of the NdF₃/LiF mass ratio on the concentration of Nd(II) in NdF₃-LiF melts with excessive spheric metal Nd and on the current efficiency for Nd extraction from the NdF₃-LiF-Nd₂O₃ melts by electrolysis has also been researched.

2. Experiments

All the experiments were carried out in a graphite crucible or a Mo crucible, which was placed in an airtight stainless steel reactor with cooling water. The reactor was heated in an electric furnace to the designated temperature. A K-type thermocouple with an accuracy of ±1 K was used to measure the experiments temperature. Before the experiments, the chemicals including LiF (aladdin, 99.9% purity), NdF₃ (aladdin, 99.9% purity) and Nd₂O₃ (aladdin, 99.9% purity) were pretreated under argon gas (99.99% purity) according to the following steps: First, each chemical was placed in a graphite crucible (spectrographic purity) and then heated to 673 K and remained for 8 h to remove traces of moisture in the airtight stainless steel reactor under the pressure of 0.016 MPa. Then the chemical was further heated to 1073 K and maintained for 2 h at 1073 K under argon gas atmosphere with a flow rate of 1.2 L/h. At last, the chemical was taken out from the reactor after cooling to room temperature (about 298 K) and stored in a glove box under argon gas with both O₂ and H₂O levels below 1 ppm before using.

During the electrochemical measurements, the three-electrode system was used, in which the working electrode and reference electrode were W wires (99.9%, dia.1 mm), the auxiliary electrode was graphite rod (spectrographic purity, dia.7 mm). And the electrochemical experiments were performed by using a PAR-STAT2273 (PAR-Ametek Co, Ltd.) with a PowerSuite software package. For all the electrochemical experiments, the potentials were transferred to Li⁺/Li according to literatures⁷⁾. In the electrolysis experiments, a two-electrode system was used, in which the anode was graphite plate (6 mm × 25 mm × 50 mm spectrographic purity) and the cathode was W plate (2 mm × 8 mm × 55 mm). Before the experiments, all the electrodes and graphite crucibles were polished by SiC emery papers to the mirror finish, washed with distilled water, then dried under argon atmosphere.

During the experiments, the salt mixtures (NdF₃-LiF, NdF₃-LiF-Nd and NdF₃-LiF-Nd₂O₃) in the graphite crucible or the Mo crucible located in the airtight stainless steel reactor were heated to 673 K and remained for 2 h to remove traces of moisture under the pressure of 0.016 MPa. Then the salt mixtures were further heated to the designated temperature in 5 h under argon gas atmosphere with a flow rate of 1.2 L/h.

3. Results and Discussion

3.1 Nd(III) ions electrochemical reduction processes

In Fig. 1, the cyclic voltammograms (CVs) on the W electrode were measured to investigate the reduction processes of Nd(III) ions in the NdF₃-LiF melts. Compared with the background CV in LiF melts in Fig. 1(a), the CVs in the NdF₃-LiF (50:1 mass ratio) melts in Fig. 1(b) and NdF₃-LiF (50:3 mass ratio) melts with excessive spheric metal Nd(0) in Fig. 1(c) have shown two reduction peaks and two corresponding oxidation peaks. As we reported in the previous work^7,8), the Nd(III) ions in fluoride melts were reduced in two steps, i.e., Nd(III)→Nd(II) and Nd(II)→Nd(0), indicating that Nd(II) was stable in NdF₃-LiF melts.

Fig. 1

The cyclic voltammograms recorded on W electrode (a) in LiF melts (b) in NdF₃-LiF (50:1 mass ratio) melts (c) in NdF₃-LiF (50:3 mass ratio)-Nd (excess) melts with 100 mV/s and an immersion area of 0.53 cm⁻² at 1323 K.

3.2 NdF₂ formation in NdF₃-LiF melts with excessive spheric metal Nd

Equilibrium experiments were carried out in NdF₃-LiF melts with excessive spheric metal Nd (compared with Nd(III) ions amount in the melts) in Mo crucible at 1323 K to investigate NdF₂ concentration in the melts. The Mo crucible with the above mixture was situated in an airtight stainless steel reactor and heated to 1323 K for 2 h under argon gas atmosphere. Then the mixture was cooled to room temperature (about 298 K) naturally and taken out from the crucible under argon gas atmosphere. The surface morphology of the spheric metal Nd was compared before and after experiments. The mass loss of the spheric metal Nd was calculated. The phase composition of the melts at different positions shown in Fig. 2 was analyzed by XRD to investigate NdF₂ concentration caused by the comproportionation reaction between Nd(III) and Nd(0).

Fig. 2

Schematic of equilibrium experiments including the different melts positions for XRD analysis: 1--melts away from the spheric metal Nd; 2--the melts between the spheric metal Nd and Mo crucible; 3--melts on the spheric metal Nd surface; 4--Mo crucible; 5--NdF₃-LiF melts; and 6- spheric metal Nd.

For the mixture of NdF₃-LiF (7:3 mass ratio, total mass of 12.86 g) with excessive spheric metal Nd (5.8 g), the geometric shape of metal Nd has changed from its initial sphere to semiellipse after the equilibrium experiments shown in Fig. 3. And the mass of metal Nd decreased by 35 mass% (2.03 g). The melts phase composition at different positions including on the metal Nd surface and far away from the metallic Nd surface has presented NdF₂ phase according to Fig. 4. The average NdF₂ concentration in the melts was about 45.5% (mass concentration) by calculation according to 2Nd(III) + Nd(0) = 3Nd(II) reaction.

Fig. 3

(a) Initial spheric metal Nd surface morphology before the equilibrium experiments, (b) metal Nd surface morphology and (c) metal Nd side morphology after the equilibrium experiments in the NdF₃-LiF (7:3 mass ratio, total mass of 12.86 g) melts with Nd (5.8 g) at 1323 K.

Fig. 4

XRD patterns of the melts (a) at position 1 of Fig. 2; (b) at position 2 of Fig. 2; (c) at position 3 of Fig. 2 after the equilibrium experiments in the NdF₃-LiF (7:3 mass ratio, total mass of 12.86 g) melts with Nd (5.8 g) at 1323 K.

For the mixture of NdF₃-LiF (9:1 mass ratio, total mass of 10 g) with excessive spheric metal Nd (6 g), the geometric shape of metal Nd has changed from its initial sphere to semiellipse after the equilibrium experiments shown in Fig. 5, and the mass of metal Nd decreased by 23 mass% (1.38 g). The melts phase composition at different positions as shown in Fig. 2 has presented NdF₂ phase according to Fig. 6. The average NdF₂ concentration in the melts was about 36.4% (mass concentration) by calculation according to 2Nd(III) + Nd(0) = 3Nd(II) reaction.

Fig. 5

(a) Initial spheric metal Nd surface morphology before the equilibrium experiments, (b) metal Nd surface morphology and (c) metal Nd side morphology after the equilibrium experiments in NdF₃-LiF (9:1 mass ratio, total mass of 10 g) melts with Nd (6 g) at 1323 K.

Fig. 6

XRD patterns of the melts (a) at position 1 of Fig. 2; (b) at position 2 of Fig. 2; (c) at position 3 of Fig. 2 after the equilibrium experiments in NdF₃-LiF (9:1 mass ratio, total mass of 10 g) melts with Nd (6 g) at 1323 K.

The above results revealed that NdF₂ could form and stably exist because of the comproportionation reaction between Nd(III) and metal Nd(0). As NdF₃-LiF melts composition changed from 7:3 mass ratio to 9:1 mass ratio, the metal Nd consumption decreased from 35 mass% to 23 mass%, the mass ratio of the consumed Nd with NdF₃ in NdF₃-LiF melts (i.e., $\Delta m_{Nd}/m_{NdF_3}$) decreased from 0.225 to 0.153, and NdF₂ mass concentration in the melts also decreased from 45.5% to 36.4%.

According to the Ref. 9), as NdF₃-LiF mass ratio increased from 7:3 to 9:1, Li⁺ ions concentration in NdF₃-LiF melts decreased and less free Nd(III) ions were released from more stable NdF₆³⁻ ions. So the comproportionation reaction of 2Nd(III) + Nd(0) = 3Nd(II) was shifted to the left hand side. Therefore less NdF₂ was formed and higher current efficiency could be obtained.

3.3 Current efficiency for Nd extraction from NdF₃-LiF-Nd₂O₃ melts

In order to explore the relationship between the NdF₂ concentration and the current efficiency for Nd extraction, electrolysis experiments were performed in the NdF₃-LiF melts (500 g) with various NdF₃:LiF mass ratio of 9:1, 8.5:1.5, 8:2 and 7:3 with Nd₂O₃ (2% mass concentration) at 1323 K, 1373 K and 1423 K, respectively. The current density was selected as I_anode = 3700 A·m⁻² and I_cathode = 18500 A·m⁻², and the polar distance was 0.056 m during the electrolysis experiments. After the electrolysis experiments, the current efficiency was calculated in term of eq. (1) and shown in Fig. 7.   

\[\eta = \frac{W}{CIt} \times 100\%\](1)

$\eta$: current efficiency; W: the amount of Nd, g; I: the average current, A; t: time, h; C: electrochemical equivalent, g·(A·h)⁻¹.

Fig. 7

Plots of current efficiency for Nd extraction from NdF₃-LiF melts with Nd₂O₃ (2%, mass concentration) versus electrolyte composition and temperature. The experiments condition: current densities for anode and cathode are I_anode = 3700 A·m⁻² and I_cathode = 18500 A·m⁻², respectively, the polar distance is 0.056 m.

According to Fig. 7, the current efficiency increased in different NdF₃:LiF mass ratio conditions as the temperature increased from 1323 K to 1423 K. This was because the metal Nd obtained in the cathode was subject to aggregate to a bigger sphere from lots of smaller particles, as shown in Fig. 8. Therefore the smaller specific surface area of the Nd at 1423 K greatly reduced both of the consumption of metal Nd and NdF₂ concentration in the melts by the comproportionation reaction.

Fig. 8

Cathode products (metal Nd) obtained from electrolysis experiments in the NdF₃-LiF melts (9:1 mass ratio, the mass of 500 g) with Nd₂O₃ (2% mass concentration) at (a) 1423 K, (b) 1373 K and (c) 1323 K.

As NdF₃-LiF mass ratio increased from 7:3 to 9:1, the current efficiency for Nd extraction increased from about 3% to 7% at 1323 K, from 5% to 35% at 1373 K and from 5% to 50% at 1423 K, respectively. At the same temperature, the NdF₃:LiF mass ratio had a significant effect on the current efficiency, especially at 1423 K. This was because the concentration of NdF₂ decreased in the melts as the NdF₃:LiF mass ratio changed from 7:3 to 9:1, which was in good agreement with our previous results. And the electrolyte composed of NdF₃-LiF (9:1 mass ratio) with Nd₂O₃ (2%, mass concentration) at 1423 K has presented the highest current efficiency of about 50%. Currently, the industrial current efficiency for Nd extraction by electrolysis was about 70%～80%, higher than our experiment value. This was because that the electrode distance, current density and temperature in our experiments were different from the industrial conditions.

4. Conclusions

The electrochemical reduction processes of Neodymium ions in NdF₃-LiF melts with or without excessive spheric metal Nd were measured. The results indicated that neodymium ions in the fluoride melts were reduced in two steps, i.e., Nd(III)→Nd(II) and Nd(II)→Nd(0). NdF₂ could be formed by the comproportionation reaction between Nd(III) and Nd(0) and stably exist in NdF₃-LiF melts containing metal Nd(0). NdF₂ mass concentration in the melts decreased from 45.5% to 36.4% with the increase of NdF₃-LiF mass ratio from 7:3 to 9:1 in NdF₃-LiF melts containing excessive spheric metal Nd, which resulted in a higher current efficiency during the electrolysis. And the highest current efficiency of about 50% for Nd extraction has been obtained by electrolysis in NdF₃-LiF (9:1 mass ratio) melts with Nd₂O₃ (2%, mass concentration) at 1423 K.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (No. 51274102).

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